The present invention relates to the technical field of an image recording method and an image recording apparatus that use two-dimensionally arranged light sources such as a combination of a two-dimensional spatial light modulator and a light source.
In more detail, the present invention relates to an image recording method and an image recording apparatus that make it possible to change resolution without using a zoom lens or a plurality of focusing optical systems during image recording that uses two-dimensionally arranged light sources.
Also, the present invention relates to an image recording method and an image recording apparatus that make it possible to correct aberration in an optical system and various kinds of errors in the optical system without using a zoom lens or a plurality of focusing optical systems during image recording that uses two-dimensionally arranged light sources.
Further, the present invention relates to an image recording method and an image recording apparatus that make it possible to suitably correct shading during image recording that uses two-dimensionally arranged light sources. Therefore, when applied to the printing field or the like, for instance, the image recording method and the recording apparatus make it possible to create a printing plate having an accurate dot area ratio.
Mainly used in a digital image exposure system utilized in various types of printers and the like is a so-called laser beam scan exposure (raster scan) for two-dimensionally exposing a recording medium with a laser beam modulated in accordance with an image to be recorded by deflecting the laser beam in a main scanning direction while relatively moving the recording medium and an optical system in an auxiliary scanning direction perpendicular to the main scanning direction.
In contrast to this, in recent years, there have been proposed various kinds of digital image recording apparatuses that use spatial light modulators (SLMs) having two-dimensional pixel arrangement, such as a liquid crystal display (hereinafter referred to as the “LCD”), a micromirror array (hereinafter referred to as the “MMA”) that is called a digital micromirror device™ (DMD™), and the like that are used as display means in displays, monitors, and the like.
In such image recording apparatuses, image recording is basically performed by exposing a recording medium through the projection/focusing of an image formed by a spatial light modulator on the recording medium.
As an example of image recording using the MMA, FIGS. 30A to 30C show the outline of image recording disclosed in U.S. Pat. No. 5,049,901 B, EP 0992350A1 A, and the like.
As is publicly known, an MMA 100 is a two-dimensional spatial light modulator constructed by two-dimensionally disposing a plurality of micromirrors (hereinafter referred to as the “mirrors”) 102 that are capable of being modulated (activated/deactivated) through independent rocking. Also, this MMA 100 performs image recording by focusing light emitted from an unillustrated light source and reflected by an activated mirror 102 (in an image recording state) on a recording medium Pt using a focusing optical system 104.
In the example shown in FIGS. 30A to 30C, the recording medium Pt is conveyed in a scanning direction (direction shown by the arrow in FIGS. 30A to 30C) that coincides with one of pixel array directions (directions in which the mirrors 102a to 102c are disposed in FIGS. 30A to 30C) of the MMA 100.
In FIG. 30A, among the mirrors of the MMA 100, the mirror 102a is activated and other mirrors 102 are deactivated. Therefore, only light reflected by the mirror 102a is focused on the recording medium Pt and an image is recorded at this position (shaded position).
When the recording medium Pt is conveyed and the position, at which the image has been recorded by the mirror 102a, moves, the mirror 102a is deactivated and only the mirror 102b is activated in accordance with this movement, as shown in FIG. 30B. By doing so, the image is recorded at the same position on the recording medium Pt. When the recording medium Pt is further conveyed, the mirror 102b is deactivated and only the mirror 102c is activated, as shown in FIG. 30C. By doing so, the image is recorded at the same position.
That is, with this image recording method, the image displayed by the MMA 100 is moved (shifted) in the scanning direction by switching an image display by the MMA 100 in accordance with the conveyance of the recording medium Pt. By doing so, the image is made to track and remain stationary on the conveyed recording medium Pt. As a result, two-dimensional image recording is performed through multiplex exposure by the plurality of mirrors 102.
Note that in an image recording apparatus that forms an image using an optical system like this constructed from a light source and a spatial light modulator or using light sources arranged in a two-dimensional manner (these light sources will be hereinafter collectively referred to as the “two-dimensionally arranged light sources”) and projects/focuses this image on a recording medium, the resolution of an image to be recorded is determined by the resolution (pixel pitch) of the two-dimensionally arranged light sources and the magnification of a focusing optical system.
Therefore, in order to perform image recording at a plurality of resolutions that are not in multiple relation to each other (at 2540 dpi, 2438 dpi, and 2400 dpi, for instance), it is required to prepare a zoom lens or focusing lenses whose number is determined in accordance with the resolution of an image to be recorded. As a result, the apparatus construction becomes complicated and this construction is disadvantageous from the viewpoint of cost and space.
Also, in the case where the resolution of the two-dimensionally arranged light sources or the magnification of the optical system deviates from a design value, there occurs a problem that the resolution of an image to be recorded becomes different from a design value and it is impossible to correct this error in the resolution.
The error in resolution like this also occurs in a like manner even in the case where there exists an error in the speed of the main scanning or the auxiliary scanning, in the case where there occurs an error in the size of a recording medium or a machine part due to an environmental fluctuation concerning the temperature, humidity, or the like, in the case where there exists an error in the diameter of a drum if there is used a drum scanner, and in other similar cases.
Also, an image of the two-dimensionally arranged light sources projected on a recording medium is distorted due to the distortion aberration (of barrel type, pincushion type, or the like) possessed by an optical system. Therefore, there occurs an error in the position of each pixel, which results in the occurrence of stripe-shaped unevenness in an image, blur in an edge portion, and the like. As a result, there is degraded image quality. Further, there is a case where an image is distorted because a recording medium is held on a round surface and such image distortion is a problem that also occurs in a like manner in the case where there is recognized irregularity in the disposal of the two-dimensionally arranged light sources.
Also, in the image recording apparatus described above that forms an image using the two-dimensionally arranged light sources and projects/focuses this image on a recording medium, there is a case where there occurs an error in the focusing position of each pixel, an error in the size of each pixel, an error in the light quantity of each pixel, and the like on a focusing surface due to various factors (these errors will be hereinafter collectively referred to as the “shading”).
For instance, as is publicly known, the accuracy of a focusing optical system tends to be reduced in a direction from an optical axis to a peripheral portion. As a result, the focusing position of each pixel is shifted in accordance with the position of the pixel, which causes a microscopic error in the size of an image and a local fluctuation in an image area ratio. Also, the size error of each pixel focused on the recording medium Pt is increased toward the peripheral portion (in usual cases, the pixel size is increased). As a result, the microscopic image size error occurs in a like manner, which causes a local fluctuation of the image area ratio in a like manner. For instance, a local fluctuation of the image area ratio like this becomes the locality of a dot area ratio (local fluctuation of the dot area ratio) in the case of a printing use.
Further, the light quantity on the recording medium Pt also tends to be reduced in the direction from the optical axis to the peripheral portion, which causes the unevenness of an exposing amount (=unevenness of a density) and the like.
It is possible to correct the shading like this by locally changing the exposing amount.
However, in the case of image recording that uses the two-dimensionally arranged light sources, it is required to control the exposing amount for each pixel in order to correct the shading, but it is substantially impossible to adjust the light quantity for each pixel. Therefore, the exposing amount control for the shading correction is necessarily performed through pulse modulation, so that it is required to perform very high-speed modulation. This means that the realization of the shading correction is difficult.
Also, the shading correction through exposing amount control is basically a correction where the light quantities of all pixels are corrected to be identical with the smallest light quantity of the pixels. This results in a situation where the exposing light is necessarily wasted and each light source is required to have higher output performance. As a result, an increase in cost is inevitable.